The present invention relates to a method for coupling two mutually isolated optical fibers to a third optical fiber.
It would be highly desirable, for a bar code reader, to be able to inexpensively manufacture a coupling which joined two mutually isolated optical fibers to third optical fiber. For example, the two mutually isolated fibers could be connected to a light source and a light detector respectively, and the union fiber could be used both to illuminate and to detect light reflected from a bar code pattern.
Optical couplers fall into two main categories: integrated and discrete. Integrated optical couplers are growths or diffusions into a planar material which act as areas to specifically direct optical information. A three port integrated optical coupler allows optical information to be guided within these diffused or grown "walls". The main disadvantages of these couplers are two-fold. First, the material required to generate these planar structures is of high cost. Secondly, for long path lengths, the ability to pattern the coupler in a plane material becomes very difficult. The solution to this problem has been to couple discrete optical fibers to the planar coupler. The cost incurred in producing a reliable joint between the discrete fibers and the planar coupler is high.
Discrete optical fibers can be coupled in a variety of ways. For example, a bundle of fibers can be inserted into a cup which has a high optical reflectance. In such a case, all light which enters the cup from any or all of the inserted optical fibers will be directed with equal strength back along all of the fibers. Such a design does not allow any two fibers to be optically isolated from each other since all fibers are coupled optically with equal efficiency.
Optical couplers which isolate two fibers, while coupling both to a third fiber, have been made using glass fibers by the use of a heat funsion process. In practice, two fibers are butted endwise against the end of a similar fiber and subsequently heated until a mechanical and optical joint is formed. This technique has the disadvantage that the ends of the optical fibers must be cut and polished before fusing, so that the coupler will have high optical efficiency. This procedure is costly due to the time and equipment required. Furthermore, this process has not been successful when plastic optical fibers are used thereby requiring the use of a fragile glass coupler.
Thus, at present there is no known inexpensive method for joining a mutually isolated pair of optical fibers to a third optical fiber while minimizing the optical loss.
It is thus an object of the present invention to provide a method for inexpensively joining two mutually isolated optical fibers to a third optical fiber while minimizing the optical loss.
It is a further object of the present invention to provide a coupler whereby two mutually isolated optical fibers can be joined to a third optical fiber with minimum loss.
It is a further object of the present invention to provide a method for joining two isolated optical fibers to a third optical fiber with minimum optical loss, without any requirement for polishing the ends of the optical fibers to be joined.
It is a further object of the present invention to provide a method for coupling two mutually isolated optical fibers to a third optical fiber while incurring minimum optical loss, wherein the fibers used may be plastic optical fibers.
It is a further object of the present invention to provide a method for assembling a coupling between two mutually isolated optical fibers and a third optical fiber with minimum loss, which can be rapidly and accurately performed by unskilled workers.
The majority of the optical fiber art has been directed to fibers suitable for high data rates for communications. However the problems presented by such applications are very different from the relevant problems and possibilities in such applications as those to which the present invention is described.
In particular, low loss through the coupling is not an absolute necessity in a system for a barcode reader, but simply an economic factor, since greater loss can be compensated by using a more intense illumination source. By contrast, communications fiber systems are often required to reduce insertion loss to an extremely low level, if a particular system is to be possible at all.
Similarly, the problems of mode conversion, depolarization, etc., which are so important in design of couplers for communications fibers, are of no concern in the present case.